A group of carcinogenic compounds with the general formula RR′NNO, where R and R′ are side groups with a variety of possible structures. Nitrosamines are a component of cigarette smoke and can be formed in the acidic environment of the stomach by the combination of nitrites (used as a food preservative) and amines; they also occur as contaminants of certain foods. Nitrosamines cause cancer in a number of organs, particularly in the liver, kidneys, and lungs. An example of a nitrosamine is N-nitrosodimethylamine, which has two methyl side groups (CH₃–).